1. Originally Posted by Handeman
I'm not sure I have all the figures right, but assuming a ~29 lbs, or 13kg fincan and it goes from 20 m/s to 6 m/s or 14m/s to zero in 0.1 seconds when hitting the end of the shock cord. The acceleration would be 14/0.1squared) or 1400, your formula would be F = 13 * (14/(0.1 *0.1)) = 13* (14/ 0.01) = 13 * 1400 = 18,200kg or 40124.132 lbs of force. It would be much more then 41 lbs.

That's why I say you almost never see the failures in L1 or smaller L2. Assuming a 5 lbs fincan or 2.27kg, the force would be 2.27 * 1400 or 3178kg or 7006.291 lbs, which is much easier to handle. a 2.5 lb. fincan would only impart about 3500 lbs of force.
Your error is in squaring 0.1 seconds. Acceleration is dv/dt, so 14 m/s divided by 0.1 second which is 140 m/s^2, rather than 1400.

Steve Shannon

2. Force is mass times acceleration, so 13 kg x 140 m/s^s = 1,820 Newtons, which is about 409 lbf.

Steve Shannon

3. Originally Posted by Steve Shannon
Your error is in squaring 0.1 seconds. Acceleration is dv/dt, so 14 m/s divided by 0.1 second which is 140 m/s^2, rather than 1400.

Steve Shannon
got it. It all depends on how fast it stops. That will be longer with a nylon shock cord because it takes time to stretch. Kevlar would shorten the deceleration time.
Last edited by Handeman; 29th December 2016 at 06:52 PM.

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Steve I also remembered F = M X A, but wanted to spot check myself. What do you think this website is calculating? http://www.smartconversion.com/unit_...alculator.aspx
Last edited by mlrtime99; 29th December 2016 at 06:52 PM.

5. Originally Posted by Handeman
I was wondering about that. The acceleration is 14m per second squared. Since the total time of the acceleration (deacceleration) is .1 seconds, I squared that. Why isn't that right? I'm not saying it's right, I just don't quite understand why it's not.
Because those are units, not an equation.
Here are the equations:
F = m * a
a = (v1 - v2)/t where t is time.
So, F = 13 kg * (20 m/s - 6 m/s)/0.1 second

Steve Shannon

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Geez, it's sad to see how much physics has left my brain. A little refresher - http://www.physicsclassroom.com/clas...lse-Connection

7. Originally Posted by Steve Shannon
Because those are units, not an equation.
Here are the equations:
F = m * a
a = (v1 - v2)/t where t is time.
So, F = 13 kg * (20 m/s - 6 m/s)/0.1 second

Steve Shannon
Let me finish the calculations:
a = 20 m/s - 6 m/s)/0.1 second = 140 m/s/s, which is a little more than 14 gees, which seems reasonable.
So, F = 13 kg * 140 m/s/s = 1820 kg*m/s/s or 1820 Newtons
1 N = 0.225 lbf., so 1820 N * 0.225 lbf/N = 409.5 lbf., which also seems reasonable.

8. Kevlar would shorten the deceleration time.
Kevlar is strong but not very elastic. 6-8% stretch is all it has usually before it snaps, and that is at full rated load.

I have gone away from Kevlar and now use tubular nylon. Z-folds are also done to help absorb the energy. I think the extra delay and more controlled way they apply the force from the chute to the airframe enables the airframe to be more pointed at the chute when the full load is taken up by the harness. That lowers the probability of flossing (zippering) the airframe, and/or reducing the stressing of the harness where it exits the airframe.

Nice project!

9. Originally Posted by OverTheTop
Kevlar is strong but not very elastic. 6-8% stretch is all it has usually before it snaps, and that is at full rated load.

I have gone away from Kevlar and now use tubular nylon. Z-folds are also done to help absorb the energy. I think the extra delay and more controlled way they apply the force from the chute to the airframe enables the airframe to be more pointed at the chute when the full load is taken up by the harness. That lowers the probability of flossing (zippering) the airframe, and/or reducing the stressing of the harness where it exits the airframe.

Nice project!
Exactly.

10. I use 5/8 inch milspec tubular nylon with a breaking strength of 2500 lbs. I like that because it has a good balance of strength and some stretch while still remaining relatively light. It will be attached to a Kevlar harness epoxied to the motor mount tube.

11. Sanding sanding sanding. Today, I had some time to trim off the excess drain sleeve and shaved it down to the wood on the edges. That was then sanded back to the original diameter and checked with the other mount tube and airframe tube. I got the total radius of the centering rings nice and even at 4 checkpoints to within 0.03 inches all around. When I get back tonight, I'll post pics here and on my build document.

Next up will be creating the parade bay bulkheads. Same method as the rings, just no drilling the middle hole.

12. Got a second TAP on board to help advise where needed. Excited to see this proceed. I'm targeting the next MWP for the launch.

13. The centering rings were rough-cut from the drain sleeve using a utility knife. Here's the first cutout pass. Sloppy, I know, but it gets better. :-P

I made a second pass with the knife to carefully shave off the overhanging epoxied fabric on the OD and ID of the rings. Some light sanding was needed to get a good fit on the motor mount. I use a PVC pipe wrapped with 60 grit sandpaper. Simply running the sanding pipe in circles inside the ring while rotating the ring 1/4 turn every 5 seconds or so give a nice, even sand.

Each ring was checked for fit and measured. The four centering rings all came out to withing a variance of 0.04" total width from ID of the motor mount to OD of the ring when measured at 4 points on each ring. I'm happy with that.

The mylar sheet wrap made the rings a bit smooth, so they were roughed up with a quick pass of the 60 grit sandpaper. Also, after this picture, I roughed up the motor mount tube with light sanding using 320 grit sandpaper just enough to cut into the glassine layer.

Next up is to epoxy on the centering rings and the reinforcing splines. I won't epoxy the top centering ring at this time. Leaving that off for now will allow me to go in and filet the splines after inserting into the booster tube.
Last edited by mccordmw; 4th January 2017 at 03:08 PM.

14. I like the patriot (any flavor). Good Luck!!!

fm

15. Building up the fincan this weekend. The goal is to use axial reinforcement so all the thrust will be transferred up through the structure to the bulk plate of the payload bay. Should make it very sturdy.

I've done quite a few scratch builds of varying power levels. To keep my build clear in my head, I usually fully diagram out what I will do. Here's what's planned for this build. I'll try to remember to take pics at each step as I'm doing them this weekend. I tend to get into the build and forget, though.

1. Fiberglassed centering rings are done. MMT tube is sanded to cut the glassine and then wiped down with rubbing alcohol to remove the dust. Reinforcing plywood splines are all cut to length.

2. JB Weld applied to the bottom of the MMT, and the bottom ring is slid on flush to the end of the MMT. After that cures, the short reinforcing splines are tacked on with JB Weld, and the 2nd centering ring is epoxied to the MMT and to the top of the reinforcements. Once cured, small filets are added to the reinforcements.

3. The middle centering ring is epoxied on using the fins (not shown) to measure the proper height. The CR should bump flush against the leading fin tab edge. This is held with tape until cured.

4. Like before, the top reinforcing splines are now epoxied to the MMT and fileted once they're set.

5. A 1" x 36" kevlar strap is epoxied to two sides of the MMT to act as an attachment bridle for the shock cord. 12" on the MMT both sides.

6. To insert the fincan, I leave the top CR off for now. This will let me go in and filet the splines to the airframe for maximum support. For now, add JB Weld up the sides of the frame where the splines attach and where the middle CR will rest just above the fin slots. Slide the slotted, fiberglassed booster frame down the fin can to about where the next CR is.

7. Add JB Weld to the airframe just below the fin slots for the next CR. Continue inserting to just before the next CR.

8. Add the last JB Weld for the bottom CR and finish sliding in. I like to add strap ties to the bottom to make sure I have good contact while curing.

9. Next day, come in and add filets to the top splines. When cured, drill small holes near the rear of the tube to inject some Rocketpoxy for adding a filet to the rear splines. When all finished. Epoxy on the top CR to the MMT, splines, and airframe.

10. Then it will be on to fin attachment. But I'm a ways off from that.

Not shown are the blocks of wood I added to the bottom CR as an attachment point for the bottom 1515 rail guide.

16. ## L3 Winter Build Thread - 3/4 Scale PAC-3 Patriot

Very thorough plan. I know this is getting ahead, but what is your plan for internal fin filets?

17. Originally Posted by BDB
Very thorough plan. I know this is getting ahead, but what is your plan for internal fin filets?
The fin slots have two points sanded with my dremel conical sanding tip. That opens up a space to inject thinned rocket proxy. The second point t is to check my work.

Do you self a favor and get this for \$8.

http://s.aliexpress.com/VFreMbIn

It's a miracle tool. Stick it in the second hole and visually check you epoxy work.

18. Join Date
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Posts
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Originally Posted by mccordmw
The fin slots have two points sanded with my dremel conical sanding tip. That opens up a space to inject thinned rocket proxy. The second point t is to check my work.

Do you self a favor and get this for \$8.

http://s.aliexpress.com/VFreMbIn

It's a miracle tool. Stick it in the second hole and visually check you epoxy work.
Okay, that actually looks really cool. I'm not that concerned about my internal fillets, but this could come in handy a lot...

19. Originally Posted by mccordmw
Do you self a favor and get this for \$8.

http://s.aliexpress.com/VFreMbIn

It's a miracle tool. Stick it in the second hole and visually check you epoxy work.
Very cool!

20. Fin can construction was started according to my plans. Aft most ring, lower reinforcements, and the second ring were epoxied in using JB Weld. Once they're cured overnight, I'll go in with filets.

Marks were made 13 cm above the aft ring to mark where the second ring goes. Hard to see here.

JB Weld was generously applied in a cicle at the bottom of the MMT, in a circle where the second ring goes, and along the edges of the reinforcing splines.

Letting this sit to cure overnight. I'll then sand them flush to the rings and add filets.

I cut out the outline for my fins. Using the extra wood on the root, I also cut out 3 spacers to hold up the next centering ring while it cures. Those spacers were taped on to the MMT while the next centering ring was pushed down on to JB Weld.

I learned from mistakes in the past to tape on some wax paper whenever using anything to prop up a part as it cures.

While the upper ring cures, I got to work on the filets for the lower centering rings and reinforcing splines. Here's a work-in progress shot (I'm a sloppy epoxy-er).

I got all the filets for the lower parts done. I'll get a shot tonight after it's all cured.

The next step is to epoxy on all the long reinforcing splines that will reach to the top centering ring. Then it's on to cutting fin slots and fiberglassing the airframe.

22. In the future you might consider peeling the glassine layer off the motor mount tube as it allows the epoxy to soak into the tube a little, roughing the glassine with sandpaper like you did should work fine too. The pics look like the glassine is still on the tubes.

23. What do you think all this wood is doing beyond adding weight to the wrong end of the rocket???
Especially those half-width sections forward of the second CR?

Do you TAPS agree this is needed, or are they just "observing?"

24. Originally Posted by FredA
What do you think all this wood is doing beyond adding weight to the wrong end of the rocket???
Especially those half-width sections forward of the second CR?

Do you TAPS agree this is needed, or are they just "observing?"
Fred, I believe the thinner pieces are to be removed and that they are there only to keep the CR square to the airframe until the epoxy sets hence the wax paper on the ends.

25. Originally Posted by FredA
What do you think all this wood is doing beyond adding weight to the wrong end of the rocket???
Especially those half-width sections forward of the second CR?

Do you TAPS agree this is needed, or are they just "observing?"
I'm not his TAP and I don't think it's my job to criticize someone else's L3 candidate or project as long as safety isn't an issue.

26. Originally Posted by rharshberger
In the future you might consider peeling the glassine layer off the motor mount tube as it allows the epoxy to soak into the tube a little, roughing the glassine with sandpaper like you did should work fine too. The pics look like the glassine is still on the tubes.
Thanks. Yes, I sanded the glassine. Mainly because I hate peeling the stuff. When it comes off easily, it's no sweat. I've had a few MMT tubes where it comes off in bits and that drives my bonkers.

27. Originally Posted by rharshberger
Fred, I believe the thinner pieces are to be removed and that they are there only to keep the CR square to the airframe until the epoxy sets hence the wax paper on the ends.
Correct. I guess that wasn't fully clear since I didn't explicitly state I was going to remove it later. They are spacers to keep the next centering ring perfectly level. The wax paper keeps them from getting stuck. I popped them out this morning before leaving for work. They came out cleanly and the ring is nice and level.

28. Thanks for your feedback. I've been in quite a bit of contact with my two TAPs with questions and answers and commentary on this project.

As Rich observed, the half width wood pieces are temporary supports.

29. Originally Posted by mccordmw
Correct. I guess that wasn't fully clear since I didn't explicitly state I was going to remove it later. They are spacers to keep the next centering ring perfectly level. The wax paper keeps them from getting stuck. I popped them out this morning before leaving for work. They came out cleanly and the ring is nice and level.
Smart!

Steve Shannon

30. Originally Posted by mccordmw
Correct. I guess that wasn't fully clear since I didn't explicitly state I was going to remove it later. They are spacers to keep the next centering ring perfectly level. The wax paper keeps them from getting stuck. I popped them out this morning before leaving for work. They came out cleanly and the ring is nice and level.
Meaning came through loud and clear when I read it . This should turn out really solid. Won't have to worry about breaking the waiver either!

The Estes Patriot (SAM) was my first rocket built at home, and I did my L1 with the 4" Madcow version (Big SAM vv). It'll be cool to your big one as an L3.

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